1. Field of the Invention
The present invention relates to non-reciprocal circuit devices, and in particular relates to a non-reciprocal circuit device, such as an isolator and a circulator, preferably for use in microwave bands, for example.
2. Description of the Related Art
To date, non-reciprocal circuit devices, such as isolators and circulators, have characteristics such that the non-reciprocal circuit devices transmit signals only in a particular direction determined in advance and do not transmit signals in the opposite direction. Utilizing such characteristics, for example, isolators are used in transmission circuit units of mobile communication devices, such as cellular phones.
A non-reciprocal circuit device described in Japanese Unexamined Patent Application Publication No. 10-284909 is known as this type of non-reciprocal circuit device that operates in a magnetic field lower than a magnetic resonant point. The non-reciprocal circuit device described in Japanese Unexamined Patent Application Publication No. 10-284909 operates in a medium magnetic field in which magnetic permeability μ+′ uses a negative region. As the medium magnetic field is close to the magnetic resonant point, a magnetic loss is large, and an insertion loss thus increases. A non-reciprocal circuit device described in Non TDK Tech-Mag with ferrite (http://www.tdk.co.jp/techmag/ferrite/grain#54/flame54.htm) operates in a low magnetic field in which the magnetic permeability μ+′ uses a positive region and is of a waveguide type (distributed constant type). In a waveguide type isolator, a ferrite is formed to have a size that is ½ of a frequency λ, and thus the size thereof increases in the 800 MHz band or in a several GHz band. In addition, in a low magnetic field operation, a phase difference between positive and negative circularly polarized waves is generated with small magnetic permeability, and thus a line length of approximately ½ of the frequency λ is required. Accordingly, the size of the isolator increases, and it is difficult to install such an isolator in a cellular phone or the like.
It is to be noted that, in the present specification, magnetic field regions (a high magnetic field, a medium magnetic field, and a low magnetic field) are defined as follows. A region where the magnetic field is higher than the magnetic resonant point is defined as a high magnetic field; a region where the magnetic field is lower than the magnetic resonant point and the magnetic permeability μ+′ is in a negative region is defined as a medium magnetic field; and a region where the magnetic field is lower than the magnetic resonant point and the magnetic permeability μ+′ is in a positive region is defined as a low magnetic field.